netdev - Re: [PATCH 2/4] arch: Add lightweight memory barriers fast_rmb() and fast

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Date:	Mon, 17 Nov 2014 13:54:58 -0800
From:	Alexander Duyck <alexander.h.duyck@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Peter Zijlstra <peterz@...radead.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Michael Neuling <mikey@...ling.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	donald.c.skidmore@...el.com, matthew.vick@...el.com,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	Francois Romieu <romieu@...zoreil.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	nic_swsd@...ltek.com, Will Deacon <will.deacon@....com>,
	Michael Ellerman <michael@...erman.id.au>,
	Tony Luck <tony.luck@...el.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	David Miller <davem@...emloft.net>
Subject: Re: [PATCH 2/4] arch: Add lightweight memory barriers fast_rmb()
 and fast_wmb()


On 11/17/2014 12:52 PM, Linus Torvalds wrote:
> On Mon, Nov 17, 2014 at 9:18 AM, Alexander Duyck
> <alexander.h.duyck@...hat.com> wrote:
>> There are a number of situations where the mandatory barriers rmb() and
>> wmb() are used to order memory/memory operations in the device drivers
>> and those barriers are much heavier than they actually need to be.
> Ugh. I absolutely despise the name.
>
> It's not "fast". It's just limited. It's the same as "smp_*mb()", in
> that it works on cacheable memory, but it actually stays around even
> for non-SMP builds.
>
> So I think the name is actively misleading.
>
> Naming should be about what it does, not about some kind of PR thing
> that confuses people into thinking it's "better".
>
> Maybe "dma_*mb()" would be acceptable, and ends up having the same
> naming convention as "smb_*mb()", and explains what it's about.

What would you think of the name "coherent_*mb()"?  I would prefer to 
avoid dma in the name since, at least in my mind, that implies MMIO.

It also ties in well with dma_alloc_coherent/dma_free_coherent which is 
what would typically be used to allocate the memory we would be using 
the barrier to protect anyway.

> And yes, in the same spirit, it would probably be good to try to
> eventually get rid of the plain "*mb()" functions, and perhaps call
> them "mmio_*mb()" to clarify that they are about ordering memory wrt
> mmio.
>
> Hmm?
>
>                          Linus

I will work on pulling all of the coherent barrier cases out of using 
the plain "*mb()" calls first.  We need to sort that out before we could 
look at renaming the plain barrier functions.

- Alex
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